The following is from 3GPP TS 23.401, Annex J:
General description of the ISR concept
Idle state Signalling Reduction (or ISR) aims at reducing the frequency of Tracking Area Updates (TAU, in EUtran) and Routing Area Updates (RAU, in UTRAN/GERAN) procedures caused by UEs reselecting between E-UTRAN and GERAN/UTRAN which are operated together. Especially the update signalling between UE and network is reduced. But also network internal signalling is reduced. To some extent the reduction of network internal signalling is also available when ISR is not used or not activated by the network.
UMTS described already RAs containing GERAN and UTRAN cells, which also reduces update signalling between UE and network. The combination of GERAN and UTRAN into the same RAs implies however common scaling, dimensioning and configuration for GERAN and UTRAN (e.g. same RA coverage, same SGSN service area, no GERAN or UTRAN only access control, same physical node for GERAN and UTRAN). As an advantage it does not require special network interface functionality for the purpose of update signalling reduction.
ISR enables signalling reduction with separate SGSN and MME and also with independent TAs and RAs. Thereby the interdependency is drastically minimized compared with the GERAN/UTRAN RAs. This comes however with ISR specific node and interface functionality. SGSN and MME may be implemented together, which reduces some interface functions but results also in some dependencies.
ISR support is mandatory for E-UTRAN UEs that support GERAN and/or UTRAN and optional for the network. ISR requires special functionality in both the UE and the network (i.e. in the SGSN, MME and Serving GW) to activate ISR for a UE. For this activation, the MME/SGSN detects whether S-GW supports ISR based on the configuration and activates ISR only if the S-GW supports the ISR. The network can decide for ISR activation individually for each UE. Gn/Gp SGSNs do not support ISR functionality. No specific HSS functionality is required to support ISR.
NOTE. A Release 7 HSS needs additional functionality to support the 'dual registration' of MME and SGSN. Without such an upgrade, at least PS domain MT Location Services and MT Short Messages are liable to fail.
It is inherent functionality of the MM procedures to enable ISR activation only when the UE is able to register via E-UTRAN and via GERAN/UTRAN. For example, when there is no E-UTRAN coverage there will be also no ISR activation. Once ISR is activated it remains active until one of the criteria for deactivation in the UE occurs, or until SGSN or MME indicate during an update procedure no more the activated ISR, i.e. the ISR status of the UE has to be refreshed with every update.
When ISR is activated this means the UE is registered with both MME and SGSN. Both the SGSN and the MME have a control connection with the Serving GW. MME and SGSN are both registered at HSS. The UE stores MM parameters from SGSN (e.g. P-TMSI and RA) and from MME (e.g. GUTI and TA(s)) and the UE stores session management (bearer) contexts that are common for E-UTRAN and GERAN/UTRAN accesses. In idle state the UE can reselect between E-UTRAN and GERAN/UTRAN (within the registered RA and TAs) without any need to perform TAU or RAU procedures with the network. SGSN and MME store each other's address when ISR is activated.
When ISR is activated and downlink data arrive, the Serving GW initiates paging processes on both SGSN and MME. In response to paging or for uplink data transfer the UE performs normal Service Request procedures on the currently camped-on RAT without any preceding update signalling (there are however existing scenarios that may require to perform a RAU procedure prior to the Service Request even with ISR is activated when GERAN/UTRAN RAs are used together, as specified in clause 220.127.116.11 of TS 23.060 ).
The UE and the network run independent periodic update timers for GERAN/UTRAN and for E-UTRAN. When the MME or SGSN do not receive periodic updates MME and SGSN may decide independently for implicit detach, which removes session management (bearer) contexts from the CN node performing the implicit detach and it removes also the related control connection from the Serving GW. Implicit detach by one CN node (either SGSN or MME) deactivates ISR in the network. It is deactivated in the UE when the UE cannot perform periodic updates in time. When ISR is activated and a periodic updating timer expires the UE starts a Deactivate ISR timer. When this timer expires and the UE was not able to perform the required update procedure the UE deactivates ISR.
Part of the ISR functionality is also available when ISR is not activated because the MM contexts are stored in UE, MME and SGSN also when ISR is not active. This results in some reduced network signalling, which is not available for Gn/Gp SGSNs. These SGSNs cannot handle MM and session management contexts separately. Therefore all contexts on Gn/Gp SGSNs are deleted when the UE changes to an MME. The MME can keep their MME contexts in all scenarios.
Gn = IP Based interface between SGSN and other SGSNs and (internal) GGSNs. DNS also shares this interface. Uses the GTP Protocol.
Gp = IP based interface between internal SGSN and external GGSNs. Between the SGSN and the external GGSN, there is the border gateway (which is essentially a firewall). Also uses the GTP Protocol.
"Temporary Identity used in Next update" (TIN)
The UE may have valid MM parameters both from MME and from SGSN. The "Temporary Identity used in Next update" (TIN) is a parameter of the UE's MM context, which identifies the UE identity to be indicated in the next RAU Request or TAU Request message. The TIN also identifies the status of ISR activation in the UE.
The TIN can take one of the three values, "P-TMSI", "GUTI" or "RAT-related TMSI". The UE sets the TIN when receiving an Attach Accept, a TAU Accept or RAU Accept message as specified in table 18.104.22.168-1.
"ISR Activated" indicated by the RAU/TAU Accept message but the UE not setting the TIN to "RAT-related TMSI" is a special situation. By maintaining the old TIN value the UE remembers to use the RAT TMSI indicated by the TIN when updating with the CN node of the other RAT.
Only if the TIN is set to "RAT-related TMSI" ISR behaviour is enabled for the UE, i.e. the UE can change between all registered areas and RATs without any update signalling and it listens for paging on the RAT it is camped on. If the TIN is set to "RAT-related TMSI", the UE's P-TMSI and RAI as well as its GUTI and TAI(s) remain registered with the network and valid in the UE.
When ISR is not active the TIN is always set to the temporary ID belonging to the currently used RAT. This guarantees that always the most recent context data are used, which means during inter-RAT changes there is always context transfer from the CN node serving the last used RAT. The UE identities, old GUTI IE and additional GUTI IE, indicated in the next TAU Request message, and old P-TMSI IE and additional P-TMSI/RAI IE, indicated in the next RAU Request message depend on the setting of TIN.
The UE indicates also information elements "additional GUTI" or "additional P-TMSI" in the Attach Request, TAU or RAU Request. These information elements permit the MME/SGSN to find the already existing UE contexts in the new MME or SGSN, when the "old GUTI" or "old P-TMSI" indicate values that are mapped from other identities.
The information flow in Figure below shows an example of ISR activation. For explanatory purposes the figure is simplified to show the MM parts only.
The process starts with an ordinary Attach procedure not requiring any special functionality for support of ISR. The Attach however deletes any existing old ISR state information stored in the UE. With the Attach request message, the UE sets its TIN to "GUTI". After attach with MME, the UE may perform any interactions via E-UTRAN without changing the ISR state. ISR remains deactivated. One or more bearer contexts are activated on MME, Serving GW and PDN GW, which is not shown in the figure.
The first time the UE reselects GERAN or UTRAN it initiates a Routing Area Update. This represents an occasion to activate ISR. The TIN indicates "GUTI" so the UE indicates a P-TMSI mapped from a GUTI in the RAU Request. The SGSN gets contexts from MME. When the MME sends the context to the SGSN, the MME includes the ISR supported indication only if the involved S-GW supports the ISR. After the ISR activated, both CN nodes keep these contexts because ISR is being activated. The SGSN establishes a control relation with the Serving GW, which is active in parallel to the control connection between MME and Serving GW (not shown in figure). The RAU Accept indicates ISR activation to the UE. The UE keeps GUTI and P-TMSI as registered, which the UE memorises by setting the TIN to "RAT-related TMSI". The MME and the SGSN are registered in parallel with the HSS.
After ISR activation, the UE may reselect between E-UTRAN and UTRAN/GERAN without any need for updating the network as long as the UE does not move out of the RA/TA(s) registered with the network.
The network is not required to activate ISR during a RAU or TAU. The network may activate ISR at any RAU or TAU that involves the context transfer between an SGSN and an MME. The RAU procedure for this is shown in Figure above. ISR activation for a UE, which is already attached to GERAN/UTRAN, with a TAU procedure from E-UTRAN works in a very similar way.
Reference: 3GPP TS 23.401: General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access